A magneto-optical recording medium has been practically used as a rewritable optical recording medium. Information is recorded on and reproduced from a magneto-optical recording medium with a converged light beam emitted from a semiconductor laser. However, the magneto-optical recording medium has such a drawback that the reproduction properties deteriorate when the diameter of a recording bit as a recording-use domain and the interval of the recording bits become smaller with respect to the diameter of the light beam.
When the diameter of the recording bit and the interval of the recording bits become smaller with respect to the diameter of the light beam, a recording bit adjacent to a target recording bit enters into the diameter of the light beam converged on the target recording bit. As a result, individual recording bits can not be read out separately and the reproduction properties deteriorate.
A structure for solving the above drawback of the magneto-optical recording medium is proposed in “High-Density Magneto-Optical Recording with Domain Wall Displacement Detection” (Joint Magneto-Optical Recording International Symposium/International Symposium on Optical Memory 1997 Technical Digest, Tu-E-04, p. 38,39). In this magneto-optical recording medium, the first, second and third magnetic layers are layered in this order. The first magnetic layer is made of a perpendicularly magnetized film having a relatively small wall coercivity and a relatively large wall mobility compared with those of the third magnetic layer in the vicinity of a readout temperature. The Curie temperature of the second magnetic layer is set lower than the Curie temperatures of the first and third magnetic layers. According to this structure, even when the recording bit diameter and the recording bit interval are small, individual recording bits can be read out separately without lowering the readout signal level, by moving the domain wall into a region where the temperature has been rased by the irradiation of a light beam.
A method of reproducing information on the magneto-optical recording medium with the above-described structure will be explained with reference to FIG. 8.
A first magnetic layer 110, a second magnetic layer 120 and a third magnetic layer 130 are layered in an exchange coupled state. Denoting the Curie temperatures of the first through third magnetic layers in the laminated state by Tc110, Tc120 and Tc130, respectively, Tc110 and Tc120 satisfy the relationship Tc120<Tc110. In FIG. 8, the arrows show the direction of transition metal magnetic moments of the respective magnetic layers. Here, magnetic domains have already been recorded in the third magnetic layer 130, and an upwardly oriented magnetic domain and a downwardly oriented magnetic domain are present alternately in a repeated manner.
When a reproduction-use light beam 104 is irradiated and converged on such a magneto-optical recording medium from the first magnetic layer 110 side, the second magnetic layer 120 has a region heated to a temperature equal to or higher than its Curie temperature. In a region having a temperature equal to or lower than the Curie temperature, the magnetic domain information in the third magnetic layer 130 is copied to the first magnetic layer 110 through the second magnetic layer 120 by the exchange coupling. In other words, the upward transition metal magnetic moment at the front part of a region 108 irradiated with the light beam is copied as it is from the third magnetic layer 130 to the first magnetic layer 110
On the other hand, in the region heated to a temperature equal to or higher than the Curie temperature of the second magnetic layer 120 (the region located behind the light beam 104 by a movement of the medium such as a rotation of a disk substrate), since the exchange coupling between the first magnetic layer 110 and third magnetic layer 130 is cut off by the second magnetic layer 120, the domain wall in the first magnetic layer 110 is readily movable.
When the information in the third magnetic layer 130 is copied as it is to the first magnetic layer 110, a domain wall 105 is essentially formed. However, in the region where the second magnetic layer 120 has been heated to a temperature equal to or higher than its Curie temperature, since the domain wall in the first magnetic layer 110 is readily movable, the domain wall 105 moves to the most stable location. Here, considering a fact that the domain wall energy density decreases with an increase in temperature, the domain wall 105 moves to a location where the temperature is increased most by the irradiation of the light beam 104, and forms a domain wall 106.
Thus, in the magneto-optical recording medium of the above-described structure, since the domain wall can be moved by the characteristic of the second magnetic layer 120, the recording domain in the third magnetic layer 130 can be enlarged in the first magnetic layer 110. Therefore, even when the recording domain is reduced, it is possible to increase the amplitude of the readout signal from the first magnetic layer 110, thereby allowing readout of signals of a cycle less than the diffraction limit of light.
However, in the above-mentioned reproduction method, there are two types of domain movements, i.e., a domain movement from the front part and a domain movement from the rear part. Hence, there is a problem that a single domain is read out twice. Referring now to FIGS. 9 and 10, the following description will explain this point.
FIG. 9 shows a state in which an independent magnetic domain 107 formed in the third magnetic layer 130 is present at the front part of the light beam 104, the third magnetic layer 130 and first magnetic layer 110 are exchange coupled at the position of the independent magnetic domain 107, and the upward moment is copied to the first magnetic layer 110. In FIG. 9, the shaded portion of the second magnetic layer 120 is a region X where the second magnetic layer 120 is heated to its Curie temperature or a higher temperature.
In the state shown in FIG. 9, the domain wall 105 moves to the position of the domain wall 106 to enlarge the magnetic domain, and a readout magnetic domain 109 with an upward moment is formed in the region 108 irradiated with the light beam 104. Therefore, a large readout signal amplitude is obtained.
When the medium (magneto-optical recording medium) is moved relatively to the light beam 104 from the state shown in FIG. 9, a downward moment of the third magnetic layer 130 is copied to the first magnetic layer 110 upon passage of the independent magnetic domain 107 through the region X, and the moment in the readout magnetic domain 109 is also oriented downward.
Further, when the medium is moved into a state shown in FIG. 10, i.e., the independent magnetic domain 107 is located at the rear part of the region X of the second magnetic layer 120, the upward moment of the independent magnetic domain 107 in the third magnetic layer 130 is copied to the first magnetic layer 110, and a domain wall 105′ moves to the position of a most stable domain wall 106′. Thus, a readout magnetic domain 140 with an upward moment exists in the region 108 irradiated with the light beam 104.
As described above, the independent magnetic domain 107 is read out once when it is located at the front part of the region X where the second magnetic layer 120 is heated to its Curie temperature or above by the irradiation of the light beam (in the state shown in FIG. 9), and read out again when it is located at the rear part of the region X (in the state shown in FIG. 10). This phenomenon is noticeable in a relatively long recording magnetic domain where the exchange coupling between the third magnetic layer 130 and first magnetic layer 110 is stable as disclosed in “High-Density Magneto-Optical Recording with Domain Wall Displacement Detection” (Joint Magneto-Optical Recording International Symposium/International Symposium on Optical Memory 1997 Technical Digest, Tu-E-04, p. 38,39).
Thus, in a conventional magneto-optical recording medium, since a relatively long recording magnetic domain can not be read out in a stable manner, a serious problem will occur when performing recording and reproduction by a mark edge recording method in which information is recorded at a higher density.